Buried contact through fin-to-fin space for vertical transport field effect transistor

ABSTRACT

Embodiments of the present invention are directed to fabrication methods and resulting structures that provide buried contacts in the fin-to-fin space of vertical transport field effect transistors (VFETs) that connect the bottom S/D of the transistors to a buried power rail. In a non-limiting embodiment of the invention, a buried power rail is encapsulated in a buried oxide layer of a first wafer. First and second semiconductor fins are formed on a second wafer. The first wafer to the second wafer and a surface of the buried power rail in a fin-to-fin space is exposed. A buried via is formed on the exposed surface of the buried power rail. The buried via electrically couples the buried power rail to a bottom source or drain region of the first semiconductor fin.

BACKGROUND

The present invention generally relates to fabrication methods andresulting structures for semiconductor devices, and more specifically,to fabrication methods and resulting structures for a vertical transportfield effect transistor (VFET) having a buried contact in the fin-to-finspace that connects the bottom source/drain (S/D) and buried power rail.

Known metal oxide semiconductor field effect transistor (MOSFET)fabrication techniques include process flows for constructing planarfield effect transistors (FETs). A planar FET includes a substrate (alsoreferred to as a silicon slab); a gate formed over the substrate; sourceand drain regions formed on opposite ends of the gate; and a channelregion near the surface of the substrate under the gate. The channelregion electrically connects the source region to the drain region whilethe gate controls the current in the channel. The gate voltage controlswhether the path from drain to source is an open circuit (“off”) or aresistive path (“on”).

In recent years, research has been devoted to the development ofnonplanar transistor architectures. For example, VFETs employsemiconductor fins and side-gates that can be contacted outside theactive region, resulting in increased device density and some increasedperformance over lateral devices. In contrast to planar FETs, the sourceto drain current in a VFET flows through the vertical fin in a directionthat is perpendicular with respect to a horizontal major surface of thewafer or substrate. A VFET can achieve a smaller device footprintbecause its channel length is decoupled from the contacted gate pitch.

SUMMARY

Embodiments of the invention are directed to a method of formingsemiconductor devices on a wafer, wherein the method provides a buriedcontact in the fin-to-fin space of vertical transport field effecttransistors (VFETs) that connects the bottom source or drain (S/D) ofthe transistors to a buried power rail. A non-limiting example of themethod includes forming a buried power rail in a buried oxide layer of afirst wafer. First and second semiconductor fins are formed on a secondwafer. The first wafer, the second wafer and a surface of the buriedpower rail in a fin-to-fin space are exposed. A buried via is formed onthe exposed surface of the buried power rail. The buried viaelectrically couples the buried power rail to a bottom source or drainregion of the first semiconductor fin.

Embodiments of the invention are directed to a method of formingsemiconductor devices on a wafer, wherein the method provides a buriedcontact in the fin-to-fin space of VFETs that connects the bottom S/D ofthe transistors to a buried power rail. A non-limiting example of themethod includes forming a buried power rail in a buried oxide layer of afirst substrate. A second substrate is formed on the buried oxide layerand first and second semiconductor fins are formed on the secondsubstrate. A surface of the buried power rail in a fin-to-fin space isexposed. A buried via is formed on the exposed surface of the buriedpower rail. The buried via electrically couples the buried power rail toa bottom source or drain region of the first semiconductor fin.

Embodiments of the invention are directed to an integrated circuit (IC).A non-limiting example of the IC includes a first semiconductor fin anda second semiconductor fin separated by a fin-to-fin space. A buriedpower rail is encapsulated in a buried oxide layer and a buried via isformed on the buried power rail in the fin-to-fin space. The buried viaelectrically couples the buried power rail to a bottom S/D region of thefirst semiconductor fin.

Additional technical features and benefits are realized through thetechniques of the present invention. Embodiments and aspects of theinvention are described in detail herein and are considered a part ofthe claimed subject matter. For a better understanding, refer to thedetailed description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The specifics of the exclusive rights described herein are particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe embodiments of the invention are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 depicts a cross-sectional view of a front-end-of-line (FEOL)region of an integrated circuit (IC) wafer during FEOL fabricationoperations for forming vertical transport field effect transistors(VFETs) on the wafer according to one or more embodiments of theinvention;

FIG. 2 depicts a top-down view of an IC wafer after a processingoperation according to one or more embodiments of the invention;

FIG. 3 depicts a top-down view of the IC wafer 200 that illustratesadditional layers of the IC wafer formed according to one or moreembodiments of the invention;

FIG. 4A depicts a cross-sectional isometric view of an IC wafer duringfabrication operations for forming VFETs on the IC wafer according toone or more embodiments of the invention;

FIG. 4B depicts a cross-sectional isometric view of the IC wafer duringfabrication operations for forming VFETs on the IC wafer according toone or more embodiments of the invention;

FIG. 5 depicts a cross-sectional view of an IC wafer during fabricationoperations for forming VFETs on the IC wafer according to one or moreembodiments of the invention;

FIG. 6 depicts a cross-sectional view of an IC wafer after wafer bondingaccording to one or more embodiments of the invention;

FIG. 7 depicts a cross-sectional view of the IC wafer after a processingoperation according to one or more embodiments of the invention;

FIG. 8 depicts a cut-away isometric view of the IC wafer aftercompletion of FEOL, middle-of-line (MOL), and back-end-of-line (BOEL)processing operations according to one or more embodiments of theinvention;

FIG. 9 depict a cross-sectional view of an IC wafer during fabricationoperations for forming VFETs on the IC wafer according to one or moreembodiments of the invention;

FIG. 10 depicts a flow diagram illustrating a method according to one ormore embodiments of the invention; and

FIG. 11 depicts a flow diagram illustrating a method according to one ormore embodiments of the invention.

The diagrams depicted herein are illustrative. There can be manyvariations to the diagrams or the operations described therein withoutdeparting from the spirit of the invention. For instance, the actionscan be performed in a differing order or actions can be added, deletedor modified.

In the accompanying figures and following detailed description of thedescribed embodiments of the invention, the various elements illustratedin the figures are provided with two or three-digit reference numbers.With minor exceptions, the leftmost digit(s) of each reference numbercorrespond to the figure in which its element is first illustrated.

DETAILED DESCRIPTION

The present disclosure relates to providing buried contacts in thefin-to-fin space of vertical transport field effect transistors (VFETs)to connect the bottom source/drain (S/D) of the transistors to a buriedpower rail. While primarily discussed with respect to VFETs, it isunderstood in advance that embodiments of the invention are not limitedto the particular transistor architectures or materials described inthis specification. Rather, embodiments of the present invention arecapable of being implemented in conjunction with any other type oftransistor architecture (e.g., FinFETs, nanosheet FETs, etc.) usingmaterials now known or later developed.

For the sake of brevity, conventional techniques related tosemiconductor device and integrated circuit (IC) fabrication may or maynot be described in detail herein. Moreover, the various tasks andprocess steps described herein can be incorporated into a morecomprehensive procedure or process having additional steps orfunctionality not described in detail herein. In particular, varioussteps in the manufacture of semiconductor devices andsemiconductor-based ICs are well known and so, in the interest ofbrevity, many conventional steps will only be mentioned briefly hereinor will be omitted entirely without providing the well-known processdetails.

Turning now to an overview of technologies that are more specificallyrelevant to aspects of the present invention, ICs are fabricated in aseries of stages, including a front-end-of-line (FEOL) stage, amiddle-of-line (MOL) stage, and a back-end-of-line (BEOL) stage. Theprocess flows for fabricating modern ICs are often identified based onwhether the process flows fall in the FEOL stage, the MOL stage, or theBEOL stage. Generally, the FEOL stage is where device elements (e.g.,transistors, capacitors, resistors, etc.) are patterned in thesemiconductor substrate/wafer. The FEOL stage processes include waferpreparation, isolation, gate patterning, and the formation of wells,source/drain (S/D) regions, extension junctions, silicide regions, andliners. The MOL stage typically includes process flows for forming thecontacts (e.g., CA) and other structures that communicatively couple toactive regions (e.g., gate, source, and drain) of the device element.For example, the silicidation of source/drain regions, as well as thedeposition of metal contacts, can occur during the MOL stage to connectthe elements patterned during the FEOL stage. Layers of interconnections(e.g., metallization layers) are formed above these logical andfunctional layers during the BEOL stage to complete the IC. Most ICsneed more than one layer of wires to form all the necessary connections,and as many as 5-12 layers are added in the BEOL process. The variousBEOL layers are interconnected by vias that couple from one layer toanother.

The continued scaling of semiconductor devices has resulted inchallenging fabrication requirements, especially when fabricating eversmaller transistors. Advanced FEOL processes incorporate phase-shifting,optical proximity correction, and other practices to satisfy thesescaling demands, and can achieve critical dimensions (CDs) below 20 nm.However, some challenges in fabricating advanced nonplanar transistorsremain. For example, highly scaled VFET architectures often rely onreducing cell height to achieve cell area reduction. Under certainground rules, however, scaling the cell height will cause a reduction infin length, which will result in a decrease of effective gate length(Weff) per footprint area. One solution is to relocate the power rail toa bottom portion of the VFET, which can save area for longer finlengths. For example, the power rail can be pushed down (buried) intothe isolation region (e.g., a shallow trench isolation region) at thecell boundary. Doing so raises two major challenges. First, thisapproach limits the size of the power rail, which prefers a largecritical dimension (CD) for better voltage drop along wires andelectromigration. Second, the buried power rail needs a contact from thetop of the VFET to connect the power rail and bottom source/drain (S/D).The buried power rail contact will itself consume area, mitigating thearea saving benefits that would otherwise be achieved.

Turning now to an overview of aspects of the present invention, one ormore embodiments of the invention address the above-describedshortcomings of the prior art by providing a new semiconductor structureand a method for providing buried contacts in the fin-to-fin space ofVFETs to connect the bottom S/D of the transistors to a buried powerrail. Placing the buried contacts in the fin-to-fin space allows for theburied contact to be placed in a manner that is self-aligned to thefins. Advantageously, positioning a buried contact and buried power railin this manner leverages the area below the active fin without consumingvaluable top-side area, saving area for a longer fin if desired.

Turning now to a more detailed description of aspects of the presentinvention, FIGS. 1-9 depict cross-sectional views of IC wafers 100, 200,400, 500, 600, and 700 after fabrication operations in accordance withaspects of the invention. Although the cross-sectional diagrams depictedin FIGS. 1-9 are two-dimensional, it is understood that the diagramsdepicted in FIGS. 1-9 represent three-dimensional structures. To assistwith visualizing the three-dimensional features, the top-down referenceview 101 shown in FIG. 1 provides a reference point for thecross-sectional views (X-view) shown in FIGS. 1-9 . The X-view depicts aside cross-sectional view taken across the fins 102.

FIG. 1 depicts a cross-sectional view of a FEOL region of an IC wafer100 during fabrication operations for forming VFETs on the IC wafer 100according to one or more embodiments of the invention. At thefabrication stage depicted in FIG. 1 , known fabrication operations havebeen used to form fins 102 over a substrate 104, configured and arrangedas shown. The fins 102 can be formed over the substrate 104 using knownFEOL VFET fabrication techniques. While the semiconductor structure 100is shown having two fins 102 for ease of illustration, it is understoodthat any number of fins can be formed over or in the substrate 104. Forexample, the top-down reference view 101 depicts 8 fins arranged inpairs.

The fins 102 and the substrate 104 can be made of any suitablesemiconductor materials, such as, for example, monocrystalline Si,silicon germanium (SiGe), III-V compound semiconductor, II-VI compoundsemiconductor, or semiconductor-on-insulator (SOI). Group III-V compoundsemiconductors, for example, include materials having at least one groupIII element and at least one group V element, such as one or more ofaluminum gallium arsenide (AlGaAs), aluminum gallium nitride (AlGaN),aluminum arsenide (AlAs), aluminum indium arsenide (AlIAs), aluminumnitride (AlN), gallium antimonide (GaSb), gallium aluminum antimonide(GaAlSb), gallium arsenide (GaAs), gallium arsenide antimonide (GaAsSb),gallium nitride (GaN), indium antimonide (InSb), indium arsenide (InAs),indium gallium arsenide (InGaAs), indium gallium arsenide phosphide(InGaAsP), indium gallium nitride (InGaN), indium nitride (InN), indiumphosphide (InP) and alloy combinations including at least one of theforegoing materials. The alloy combinations can include binary (twoelements, e.g., gallium (III) arsenide (GaAs)), ternary (three elements,e.g., InGaAs) and quaternary (four elements, e.g., aluminum galliumindium phosphide (AlInGaP)) alloys.

In some embodiments of the invention, the substrate 104 and the fins 102can be made of a same semiconductor material. In other embodiments ofthe invention, the substrate 104 can be made of a first semiconductormaterial, and the fins 102 can be made of a second semiconductormaterial. In some embodiments of the invention, the substrate 104 andthe fins 102 can be made of silicon or SiGe. In some embodiments of theinvention, the substrate 104 is a silicon substrate and the fins 102 aresilicon germanium fins having a germanium concentration of about 10 toabout 80 percent. The fins 102 can each have a height ranging from 4 nmto 150 nm. In some embodiments of the present invention, the fins 102are formed to a height of about 60 nm, although other fin heights arewithin the contemplated scope of the invention.

In some embodiments of the invention, the substrate 104 can include aburied oxide layer 106. The buried oxide layer can be made of anysuitable dielectric material, such as, for example, a silicon oxide. Insome embodiments of the invention, the buried oxide layer 106 is formedto a thickness of about 145 nm, although other thicknesses are withinthe contemplated scope of the invention.

In some embodiments of the invention, a shallow trench isolation (STI)region 108 can be formed in the substrate 104. The STI region 108provides electrical isolation between adjacent devices (e.g., fins 102)on the substrate 104. The STI region 108 can be formed by forming atrench (not separately shown) in the substrate 104 and filling thetrench with dielectric material, such as, for example, a low-kdielectric, an oxide, a nitride, silicon nitride, silicon oxide, SiON,SiC, SiOCN, and SiBCN.

In some embodiments of the invention, each of the fins 102 can be formedon a bottom source/drain (S/D) region 110 and a top S/D region 112 canbe formed on a top surface of the fins 102. In some embodiments of theinvention, the S/D regions 110, 112 can be epitaxially grown using, forexample, vapor-phase epitaxy (VPE), molecular beam epitaxy (MBE),liquid-phase epitaxy (LPE), or other suitable processes. The S/D regions110, 112 can include semiconductor materials epitaxially grown fromgaseous or liquid precursors.

In some embodiments of the invention, the gas source for the epitaxialdeposition of semiconductor material includes a silicon containing gassource, a germanium containing gas source, or a combination thereof. Forexample, a silicon layer can be epitaxially deposited (or grown) from asilicon gas source that is selected from the group consisting of silane,disilane, trisilane, tetrasilane, hexachlorodisilane, tetrachlorosilane,dichlorosilane, trichlorosilane, methylsilane, dimethylsilane,ethylsilane, methyldisilane, dimethyldisilane, hexamethyldisilane andcombinations thereof. A germanium layer can be epitaxially depositedfrom a germanium gas source that is selected from the group consistingof germane, digermane, halogermane, dichlorogermane, trichlorogermane,tetrachlorogermane and combinations thereof. A silicon germanium alloylayer can be epitaxially formed utilizing a combination of such gassources. Carrier gases like hydrogen, nitrogen, helium and argon can beused. In some embodiments of the invention, the epitaxial semiconductormaterials include carbon doped silicon (Si:C). This Si:C layer can begrown in the same chamber used for other epitaxy steps or in a dedicatedSi:C epitaxy chamber. The Si:C can include carbon in the range of about0.2 percent to about 3.0 percent.

Epitaxially grown silicon and silicon germanium can be doped by addingn-type dopants (e.g., P or As) or p-type dopants (e.g., Ga, B, BF₂, orAl). In some embodiments of the invention, the S/D regions 110, 112 canbe epitaxially formed and doped by a variety of methods, such as, forexample, in-situ doped epitaxy (doped during deposition), dopedfollowing the epitaxy, or by implantation and plasma doping. The dopantconcentration in the doped regions can range from 1×10¹⁹ cm⁻³ to 2×10²¹cm⁻³, or between 1×10²⁰ cm⁻³ and 1×10²¹ cm⁻³.

In some embodiments of the invention, the bottom S/D region 110 is madeof silicon, while the top S/D region 112 is made of silicon germanium,or vice versa. In some embodiments of the invention, the bottom S/Dregion 110 is doped with an n-type dopant, such as phosphorus and thetop S/D region 112 is doped with a p-type dopant, such as boron, or viceversa. In some embodiments of the invention, one of the S/D regions 110,112 is a source, while the other is a drain. Current flows verticallythrough each of the fins 102 between the source and drain.

In some embodiments of the invention, contacts 114 (top contacts, fincontacts) are formed on the top S/D region 112. The contacts 114 can bemade of any suitable conductive material, such as, for example, metals(e.g., tungsten, titanium, tantalum, ruthenium, zirconium, cobalt,copper, aluminum, lead, platinum, tin, silver, gold), conductingmetallic compound materials (e.g., tantalum nitride, titanium nitride,tantalum carbide, titanium carbide, titanium aluminum carbide, tungstensilicide, tungsten nitride, ruthenium oxide, cobalt silicide, nickelsilicide), conductive carbon, graphene, or any suitable combination ofthese materials. In some embodiments of the invention, the contacts 114are electrically coupled to one or more vias 116 and lines 118 in adielectric layer 120 (also referred to as an interlayer dielectric). Thevias 116 and lines 118 can be made of any suitable conductive material,such as those for the contacts 114. The dielectric layer 120 can be madeof any suitable dielectric material, such as, for example, siliconoxide.

As further shown in FIG. 1 , a buried power rail 122 can be formed inthe buried oxide layer 106. In some embodiments of the invention, theburied power rail 112 extends beneath a bottom surface of the fins 102.The buried power rail 122 can be made of any suitable conductivematerial, such as, for example, metals (e.g., tungsten, titanium,tantalum, ruthenium, zirconium, cobalt, copper, aluminum, lead,platinum, tin, silver, gold), conducting metallic compound materials(e.g., tantalum nitride, titanium nitride, tantalum carbide, titaniumcarbide, titanium aluminum carbide, tungsten silicide, tungsten nitride,ruthenium oxide, cobalt silicide, nickel silicide), conductive carbon,graphene, or any suitable combination of these materials.

In some embodiments of the invention, a buried via 124 (buried contact,buried power rail via) is formed in the fin-to-fin space 126 betweenadjacent fins of the fins 102 (see top-down reference view 101). Notethat the buried via 124 depicted in the cross-sectional view of FIG. 1is shown by projection only to better demonstrate the positionalrelationship between the buried via 124, the fins 102, and the buriedpower rail 122. The buried via 124 can be formed of any suitableconductive material in a similar manner as the contacts 114.

FIG. 2 depicts a top-down view of an IC wafer 200 during fabricationoperations for forming VFETs on the IC wafer 200 according to one ormore embodiments of the invention. The IC wafer 200 can include fins102, bottom S/D regions 110, buried power rails 122, buried vias 124formed in a similar manner as disused with respect to FIG. 1 . As shownin FIG. 2 , a portion of the fins 102 can be arranged into cells 202having a pFET region 204 and an nFET region 206. In some embodiments ofthe invention, power tabs 208 (power vias, power contacts)) are formedon the buried power rails 122. In some embodiments of the invention, thepower tabs 208 electrically couple the buried power rails 122 to a powersource (e.g., VDD rail, not separately shown).

FIG. 3 depicts a top-down view of the IC wafer 200 that illustratesadditional layers of the IC wafer 200 formed according to one or moreembodiments of the invention. As shown in FIG. 3 , the IC wafer 200 canfurther include lines 118 (e.g., M1 metallization lines), gate contacts310, and gate 212. The gate contacts 310 can be formed in a similarmanner and from similar conductive materials as the contacts 114discussed with respect to FIG. 1 .

FIGS. 4A and 4B depict cross-sectional isometric views of an IC wafer400 during fabrication operations for forming VFETs on the IC wafer 400according to one or more embodiments of the invention. At thefabrication stage depicted in FIG. 4A, the buried oxide layer 106 isformed on a substrate 402. In some embodiments of the invention, the ICwafer 400 is a first wafer of a two-wafer bonding process (see FIGS. 6Aand 6B). The substrate 402 can be formed from same or similarsemiconductor materials as the substrate 104 discussed with respect toFIG. 1 .

In some embodiments of the invention, the buried power rail 122 isformed in the buried oxide layer 106. In some embodiments of theinvention, the buried power rail 122 is formed by depositing conductivematerials into a damascene trench (not separately shown). Conductivematerials can include metals (e.g., Ru), alloys, or other conductivematerials. In some embodiments of the invention, conductive materialsoverfill the damascene trench, forming overburdens that can be removedusing, for example, chemical-mechanical planarization (CMP).

As shown in FIG. 4B, additional dielectric material is added to theburied oxide layer 106 to encapsulate the buried power rail 122. In someembodiments of the invention, an additional 20 nm to 30 nm ofdielectrics (e.g., oxides, silicon oxides) are formed or deposited overthe buried power rail 122, although other thicknesses are within thecontemplated scope of the disclosure.

FIG. 5 depicts a cross-sectional view of an IC wafer 500 duringfabrication operations for forming VFETs on the IC wafer 500 accordingto one or more embodiments of the invention. In some embodiments of theinvention, the IC wafer 500 is a second wafer of a two-wafer bondingprocess (see FIGS. 6A and 6B). At the fabrication stage depicted in FIG.5 , known fabrication operations have been used to form fins 102, STIregion 108, and a bottom S/D region 110, configured and arranged asshown. The fins 102, STI region 108, and bottom S/D region 110 can beformed in a similar manner as described with respect to FIG. 1 .

In some embodiments of the invention, the fins 102 are formed over adielectric layer 502. In some embodiments of the invention, thedielectric layer 502 is an oxide, such as, for example, silicon oxide.In some embodiments of the invention, the dielectric layer 502 is formedover a substrate (not separately shown). The substrate can be a bulksubstrate (e.g., bulk silicon) or a silicon-on-insulator (SOI) substrateformed using known processes.

In some embodiments of the invention, a hard mask 504 is patterned overa top surface of the fins 102. The hard mask 504 can be formed using anysuitable process, such as, for example, chemical vapor deposition (CVD),plasma-enhanced CVD (PECVD), ultrahigh vacuum chemical vapor deposition(UHVCVD), rapid thermal chemical vapor deposition (RTCVD), metalorganicchemical vapor deposition (MOCVD), low-pressure chemical vapordeposition (LPCVD), limited reaction processing CVD (LRPCVD), atomiclayer deposition (ALD), flowable CVD, spin-on dielectrics, physicalvapor deposition (PVD), molecular beam epitaxy (MBE), chemical solutiondeposition, spin-on dielectrics, or other like process. The hard mask504 can be made of any suitable dielectric material, such as, forexample, a low-k dielectric, a nitride, silicon nitride, silicon oxide,SiON, SiC, SiOCN, or SiBCN. In some embodiments of the invention, thehard mask 504 is a silicon nitride hard mask.

In some embodiments of the invention, a second hard mask 506 is formedon the hard mask 504. The hard mask 506 can be formed in a similarmanner as the hard mask 504. In some embodiments of the invention, thehard mask 506 includes an oxide, such as, for example, silicon oxide,although other dielectric materials are within the contemplated scope ofthe disclosure.

As further shown in FIG. 5 , a liner 508 can be formed over the fins 102and the hard masks 504, 506. In some embodiments of the invention, theliner 508 is conformally deposited using, for example, CVD. The liner508 can be formed to a thickness of about 1 to 15 nm, although otherthicknesses are within the contemplated scope of the disclosure. In someembodiments of the invention, the liner 508 is an oxide liner, forexample, a silicon oxide liner.

In some embodiments of the invention, a second liner 510 can be formedover the first liner 508. In some embodiments of the invention, theliner 510 is conformally deposited using, for example, CVD. The liner510 can be formed to a thickness of about 1 to 15 nm, although otherthicknesses are within the contemplated scope of the disclosure. In someembodiments of the invention, the liner 510 is a nitride liner, forexample, a silicon nitride liner.

FIG. 6 depicts a cross-sectional view of an IC wafer 600 after waferbonding according to one or more embodiments of the invention. In someembodiments of the invention, the IC wafer 600 is a bonded wafer thatincludes a wafer interface 602 between the IC wafer 400 and the IC wafer500. The IC wafers 400, 500 can be bonded using known wafer bondingtechniques. Reference view 601 depicts an isometric view of the IC wafer600.

FIG. 7 depicts a cross-sectional view of the IC wafer 600 after aprocessing operation according to one or more embodiments of theinvention. In some embodiments of the invention, a buried via 124 isformed on a surface of the buried power rail 122 in the fin-to-fin space126. The buried via 124 serves to electrically couple the bottom S/D 110to a current source (e.g., a VDD rail, not shown) through the buriedpower rail 122.

In some embodiments of the invention, portions of the liners 508, 510are removed in the fin-to-fin space 126 to expose a surface of thebottom S/D region 110. Portions of the liners 508, 510 can be removedusing known processes, such as a wet etch, a dry etch, or a combinationof sequential wet and/or dry etches.

In some embodiments of the invention, the exposed portions of the bottomS/D region 110, bottom portions of the fins 102 (i.e., those portionsunder the bottom S/D region 110) and portions of the dielectric layers502, 106 at the wafer interface 602 are removed to expose a surface ofthe buried power rail 122. Portions of the bottom S/D region 110, fins102, and dielectric layers 502, 106 can be removed using knownprocesses, such as a wet etch, a dry etch, or a combination ofsequential wet and/or dry etches.

Once the surface of the buried power rail 122 is exposed, conductivematerials can be formed or deposited therein to define the buried via124. As discussed previously, the buried via 124 can include a range ofconductive materials, such as, for example, ruthenium. In someembodiments of the invention, a dielectric cap 702 is formed on theburied via 124. The dielectric cap 702 can be made from any suitabledielectric material, such as, for example, silicon oxide. The dielectriccap 702 serves to electrically isolate the buried via 124 and the buriedpower rail 122 from the gate 802 (see FIG. 8 ). In some embodiments ofthe invention, dielectric material is deposited over the IC wafer 600and recessed to define the dielectric cap 702. Reference view 701depicts an isometric view of the IC wafer 600 illustrating thepositioning of the dielectric cap 702 relative to the fins 102.

FIG. 8 depicts a cut-away isometric view of the IC wafer 600 aftercompletion of FEOL, MOL, and BOEL processing operations according to oneor more embodiments of the invention. In some embodiments of theinvention, downstream processes continue using known FEOL, MOL, and BEOLVFET fabrication techniques to create a final, operational device. Forexample, gates 802 (e.g., high-k metal gates, HKMG, including gatedielectrics and work function metals), various dielectrics (e.g.,dielectric layer 120, low-k dielectric layer 804), top S/D region 112,contacts 114, 310, and metallization layers V0 and M1 can be fabricatedusing known VFET integration processes. Other structural elements, suchas additional metallization layers (e.g., V1, M2, etc.), are omitted forclarity.

FIG. 9 depict a cross-sectional isometric view of an IC wafer 900 duringfabrication operations for forming VFETs on the IC wafer 900 accordingto one or more embodiments of the invention. FIG. 9 depicts analternative single wafer process flow which follows generally from theflow described with respect to FIGS. 4A and 4B, except for the insertionof an alignment mark 902 in the buried oxide layer 106 and/or thesubstrate 402. After additional dielectric material is added to theburied oxide layer 106 to encapsulate the buried power rail 122 (seeFIG. 4B), a second substrate 904 is bonded to a surface of the buriedoxide layer 106. The second substrate 904 can be bonded to the buriedoxide layer 106 using known wafer bonding processes. The secondsubstrate 904 can include a same or different semiconductor materialthan the substrate 402.

Once the second substrate 904 is formed, downstream FEOL, MOL, and BEOLprocesses can continue as previously described. In other words, insteadof continuing to FIG. 5 , where various fins 102 are formed on aseparate IC wafer 500 (as follows FIG. 4B), the fins 102 and otherdevice elements are formed directly on the second substrate 902. In someembodiments of the invention, the alignment mark 902 is exposed (e.g.,at the wafer edge, etc.) and leveraged to align the fins 102 and otherdevice elements over the substrate 402 and the buried power rail 122.

FIG. 10 depicts a flow diagram 1000 illustrating a method for providingburied contacts in the fin-to-fin space of VFETs to connect the bottomS/D of the VFETs to a buried power rail according to one or moreembodiments of the invention. As shown at block 1002, a buried powerrail is encapsulated in a buried oxide layer of a first wafer. In someembodiments of the invention, encapsulating the buried power railincludes forming the buried oxide layer over a substrate of the firstwafer, forming the buried power rail in a trench in the buried oxidelayer, and depositing additional dielectric material over the buriedpower rail. In some embodiments of the invention, the buried power railis electrically coupled to a current source (e.g., a VDD rail).

At block 1004, a first semiconductor fin and a second semiconductor finare formed on a second wafer. In some embodiments of the invention, thefirst semiconductor fin and the second semiconductor fin are separatedby a fin-to-fin space. In some embodiments of the invention, the secondwafer includes a second buried oxide layer.

At block 1006, the first wafer is bonded to the second wafer. In someembodiments of the invention, a wafer interface between the first waferand the second wafer is between the buried oxide layer and the secondburied oxide layer. In some embodiments of the invention, the buriedpower rail extends under the first semiconductor fin and the secondsemiconductor fin.

At block 1008, a surface of the buried power rail in the fin-to-finspace is exposed. At block 1010, a buried via is formed on the exposedsurface of the buried power rail. In some embodiments of the invention,the buried via electrically couples the buried power rail to a bottomsource or drain region of the first semiconductor fin.

The method can further include forming a dielectric cap on the buriedvia. In some embodiments of the invention, the dielectric capelectrically isolates the buried via from a gate of one or both of thefirst semiconductor fin and the second semiconductor fin.

FIG. 11 depicts a flow diagram 1100 illustrating a method for providingburied contacts in the fin-to-fin space of VFETs to connect the bottomS/D of the VFETs to a buried power rail according to one or moreembodiments of the invention. As shown at block 1102, a buried powerrail is encapsulated in a buried oxide layer of a first substrate.

At block 1104, a second substrate is formed on the buried oxide layer.At block 1106, a first semiconductor fin and a second semiconductor finare formed on the second substrate. In some embodiments of theinvention, the first semiconductor fin and the second semiconductor finare separated by a fin-to-fin space.

At block 1108, a surface of the buried power rail is exposed in thefin-to-fin space. At block 1110, a buried via is formed on the exposedsurface of the buried power rail. In some embodiments of the invention,the buried via electrically couples the buried power rail to a bottomsource or drain region of the first semiconductor fin.

The method can further include forming an alignment mark formed in oneor both of the buried oxide layer and the first substrate. In someembodiments of the invention, the alignment mark is leveraged to alignthe first semiconductor fin and the second semiconductor fin to theburied power rail.

The methods and resulting structures described herein can be used in thefabrication of IC chips. The resulting IC chips can be distributed bythe fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includes ICchips, ranging from toys and other low-end applications to advancedcomputer products having a display, a keyboard or other input device,and a central processor.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., over, below,adjacent, etc.) are set forth between elements in the followingdescription and in the drawings, persons skilled in the art willrecognize that many of the positional relationships described herein areorientation-independent when the described functionality is maintainedeven though the orientation is changed. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present invention is not intended to be limiting inthis respect. Similarly, the term “coupled” and variations thereofdescribes having a communications path between two elements and does notimply a direct connection between the elements with no interveningelements/connections between them. All of these variations areconsidered a part of the specification. Accordingly, a coupling ofentities can refer to either a direct or an indirect coupling, and apositional relationship between entities can be a direct or indirectpositional relationship. As an example of an indirect positionalrelationship, references in the present description to forming layer “A”over layer “B” include situations in which one or more intermediatelayers (e.g., layer “C”) is between layer “A” and layer “B” as long asthe relevant characteristics and functionalities of layer “A” and layer“B” are not substantially changed by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “exemplary” is used herein to mean “serving as anexample, instance or illustration.” Any embodiment or design describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. The terms “at least one”and “one or more” are understood to include any integer number greaterthan or equal to one, i.e. one, two, three, four, etc. The terms “aplurality” are understood to include any integer number greater than orequal to two, i.e. two, three, four, five, etc. The term “connection”can include an indirect “connection” and a direct “connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements such as an interfacestructure can be present between the first element and the secondelement. The term “direct contact” means that a first element, such as afirst structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements.

Spatially relative terms, e.g., “beneath,” “below,” “lower,” “above,”“upper,” and the like, are used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device can be otherwise oriented (e.g., rotated 90degrees or at other orientations), and the spatially relativedescriptors used herein should be interpreted accordingly.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

The phrase “selective to,” such as, for example, “a first elementselective to a second element,” means that the first element can beetched and the second element can act as an etch stop.

The term “conformal” (e.g., a conformal layer or a conformal deposition)means that the thickness of the layer is substantially the same on allsurfaces, or that the thickness variation is less than 15% of thenominal thickness of the layer.

The terms “epitaxial growth and/or deposition” and “epitaxially formedand/or grown” mean the growth of a semiconductor material (crystallinematerial) on a deposition surface of another semiconductor material(crystalline material), in which the semiconductor material being grown(crystalline overlayer) has substantially the same crystallinecharacteristics as the semiconductor material of the deposition surface(seed material). In an epitaxial deposition process, the chemicalreactants provided by the source gases can be controlled and the systemparameters can be set so that the depositing atoms arrive at thedeposition surface of the semiconductor substrate with sufficient energyto move about on the surface such that the depositing atoms orientthemselves to the crystal arrangement of the atoms of the depositionsurface. An epitaxially grown semiconductor material can havesubstantially the same crystalline characteristics as the depositionsurface on which the epitaxially grown material is formed. For example,an epitaxially grown semiconductor material deposited on a {100}orientated crystalline surface can take on a {100} orientation. In someembodiments of the invention of the invention, epitaxial growth and/ordeposition processes can be selective to forming on semiconductorsurface, and may or may not deposit material on exposed surfaces, suchas silicon dioxide or silicon nitride surfaces.

As previously noted herein, for the sake of brevity, conventionaltechniques related to semiconductor device and integrated circuit (IC)fabrication may or may not be described in detail herein. By way ofbackground, however, a more general description of the semiconductordevice fabrication processes that can be utilized in implementing one ormore embodiments of the present invention will now be provided. Althoughspecific fabrication operations used in implementing one or moreembodiments of the present invention can be individually known, thedescribed combination of operations and/or resulting structures of thepresent invention are unique. Thus, the unique combination of theoperations described in connection with the fabrication of asemiconductor device according to the present invention utilize avariety of individually known physical and chemical processes performedon a semiconductor (e.g., silicon) substrate, some of which aredescribed in the immediately following paragraphs.

In general, the various processes used to form a micro-chip that will bepackaged into an IC fall into four general categories, namely, filmdeposition, removal/etching, semiconductor doping andpatterning/lithography. Deposition is any process that grows, coats, orotherwise transfers a material onto the wafer. Available technologiesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE)and more recently, atomic layer deposition (ALD) among others.Removal/etching is any process that removes material from the wafer.Examples include etch processes (either wet or dry), chemical-mechanicalplanarization (CMP), and the like. Reactive ion etching (ME), forexample, is a type of dry etching that uses chemically reactive plasmato remove a material, such as a masked pattern of semiconductormaterial, by exposing the material to a bombardment of ions thatdislodge portions of the material from the exposed surface. The plasmais typically generated under low pressure (vacuum) by an electromagneticfield. Semiconductor doping is the modification of electrical propertiesby doping, for example, transistor sources and drains, generally bydiffusion and/or by ion implantation. These doping processes arefollowed by furnace annealing or by rapid thermal annealing (RTA).Annealing serves to activate the implanted dopants. Films of bothconductors (e.g., poly-silicon, aluminum, copper, etc.) and insulators(e.g., various forms of silicon dioxide, silicon nitride, etc.) are usedto connect and isolate transistors and their components. Selectivedoping of various regions of the semiconductor substrate allows theconductivity of the substrate to be changed with the application ofvoltage. By creating structures of these various components, millions oftransistors can be built and wired together to form the complexcircuitry of a modern microelectronic device. Semiconductor lithographyis the formation of three-dimensional relief images or patterns on thesemiconductor substrate for subsequent transfer of the pattern to thesubstrate. In semiconductor lithography, the patterns are formed by alight sensitive polymer called a photo-resist. To build the complexstructures that make up a transistor and the many wires that connect themillions of transistors of a circuit, lithography and etch patterntransfer steps are repeated multiple times. Each pattern being printedon the wafer is aligned to the previously formed patterns and slowly theconductors, insulators and selectively doped regions are built up toform the final device.

The flowchart and block diagrams in the Figures illustrate possibleimplementations of fabrication and/or operation methods according tovarious embodiments of the present invention. Variousfunctions/operations of the method are represented in the flow diagramby blocks. In some alternative implementations, the functions noted inthe blocks can occur out of the order noted in the Figures. For example,two blocks shown in succession can, in fact, be executed substantiallyconcurrently, or the blocks can sometimes be executed in the reverseorder, depending upon the functionality involved.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments described. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A method for forming an integrated circuit, themethod comprising: encapsulating a buried power rail in a buried oxidelayer of a first wafer; forming a first semiconductor fin and a secondsemiconductor fin on a second wafer, wherein the first semiconductor finand the second semiconductor fin are separated by a fin-to-fin space;bonding the first wafer to the second wafer at a wafer interface;exposing a surface of the buried power rail in the fin-to-fin space; andforming a buried via on the exposed surface of the buried power rail,wherein the buried via electrically couples the buried power rail to abottom source or drain region of the first semiconductor fin, the buriedvia extending through the wafer interface.
 2. The method of claim 1,wherein the second wafer comprises a second buried oxide layer.
 3. Themethod of claim 2, wherein the wafer interface between the first waferand the second wafer is between the buried oxide layer and the secondburied oxide layer.
 4. The method of claim 1 further comprising forminga dielectric cap on the buried via.
 5. The method of claim 4, whereinthe dielectric cap electrically isolates the buried via from a gate ofthe first semiconductor fin.
 6. The method of claim 1, wherein theburied power rail extends under the first semiconductor fin and thesecond semiconductor fin.
 7. The method of claim 1, whereinencapsulating the buried power rail comprises: forming the buried oxidelayer over a substrate of the first wafer; forming the buried power railin a trench in the buried oxide layer; and depositing additionaldielectric material over the buried power rail.
 8. A method for formingan integrated circuit, the method comprising: encapsulating a buriedpower rail in a buried oxide layer of a first substrate; forming asecond substrate on the buried oxide layer at a wafer interface; forminga first semiconductor fin and a second semiconductor fin on the secondsubstrate, wherein the first semiconductor fin and the secondsemiconductor fin are separated by a fin-to-fin space; exposing asurface of the buried power rail in the fin-to-fin space; and forming aburied via on the exposed surface of the buried power rail, wherein theburied via electrically couples the buried power rail to a bottom sourceor drain region of the first semiconductor fin, the buried via extendingthrough the wafer interface.
 9. The method of claim 8 further comprisingan alignment mark formed in one or both of the buried oxide layer andthe first substrate.
 10. The method of claim 9, wherein the alignmentmark is leveraged to align the first semiconductor fin and the secondsemiconductor fin to the buried power rail.
 11. The method of claim 8further comprising forming a dielectric cap on the buried via.
 12. Themethod of claim 11, wherein the dielectric cap electrically isolates theburied via from a gate of the first semiconductor fin.
 13. The method ofclaim 8, wherein the buried power rail extends under the firstsemiconductor fin and the second semiconductor fin.
 14. The method ofclaim 8, wherein encapsulating the buried power rail comprises: formingthe buried oxide layer over a substrate of the first wafer; forming theburied power rail in a trench in the buried oxide layer; and depositingadditional dielectric material over the buried power rail.